1. Field of the Invention
This invention relates in general to subsea wellhead assemblies, and specifically for an adapter having an inner profile of a wellhead housing to be placed within a larger wellhead housing.
2. Background of the Invention
When drilling subsea wells, a low pressure wellhead housing is installed with a string of conductor casing or pipe extending therefrom into the sea floor. A high pressure wellhead housing is then landed in the bore of the high pressure wellhead housing with another string casing extending therefrom to a deeper depth. Additional strings of casing that extend deeper into the subsea well, until at least one string reaches a production depth, are suspended from casing hangers landed within the bore of the high pressure housing. A tubing hanger is then landed for supporting a sting of production tubing that receives the hydrocarbons from the subsea well after the deepest string of casing is perforated.
A standard high pressure wellhead housing has either an 18¾ inch inner diameter, or a 13⅝ inch inner diameter upon which the casing hangers land upon. As is readily understood by those skilled in the art, a casing hanger designed to land within an 13⅝ inch high pressure wellhead housing cannot engage the bore of the high pressure wellhead housing as is required. Therefore, once the high pressure wellhead housing is in place, previous assemblies required using the compatible casing hanger and tubing hanger assemblies.
A drilling vessel can complete all of these tasks at once. However, sometimes drilling ships pre-drill numerous subsea wells by drilling and installing only the low pressure and high pressure wellhead housings with their associated strings of casing. This allows the drilling ship to keep a supply of the same equipment, and move quickly from one proposed subea well to another. Later a drilling vessel comes along and drills further down to install the inner casings and casing hangers, as well as performing the tasks associated with completing the subsea wells. These drilling vessels have the pressure control equipment for performing the deeper drilling and completion operations.
A drilling operator may only have 13⅝ inch casing hangers available at a particular time, or the drilling operator may realize that it may be less expensive to drill through a subsea tree assembly rather than with a drilling riser and blowout preventer (BOP) assembly. The typical subsea tree assembly cannot have an 18¾ inch casing hanger lowered therethrough to the high pressure wellhead housing, but can accommodate a 13⅝ inch casing hanger. As is also readily appreciated by those skilled in the art, a narrower drilling riser is also less expensive to use than a drilling riser for that can be used to lower a 18¾ inch casing hanger. It may be advantageous to use smaller drilling riser simply because of costs of operation rather than a scarceness of correctly-sized casing hangers. However, in previous operations the operator was left without options once a high pressure wellhead housing with an 18¾ inch inner diameter was installed.